This invention relates to a method for reducing the amount of carbon monoxide and sulfur oxides in the flue gas produced in a catalyst regenerator in a fluid catalytic cracking system.
Modern hydrocarbon catalytic cracking systems use a moving bed or fluidized bed of a particulate catalyst. The cracking catalyst is subjected to a continuous cyclic cracking reaction and catalyst regeneration procedure. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in a hydrocarbon cracking zone, or reactor, usually at a temperature of about 800.degree.-1100.degree. F. The reactions of hydrocarbons in the hydrocarbon stream at this temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are thereafter separated from the coked catalyst and are withdrawn from the cracking zone. The coked catalyst is then stripped of volatiles and is passed to a catalyst regeneration zone. In the catalyst regenerator, the coked catalyst is contacted with a gas containing a controlled amount of molecular oxygen to burn off a desired portion of the coke from the catalyst and simultaneously to heat the catalyst to a high temperature desired when the catalyst is again contacted with the hydrocarbon stream in the cracking zone. After regeneration, the catalyst is returned to the cracking zone, where it is used to vaporize the hydrocarbons and to catalyze hydrocarbon cracking. The flue gas formed by combustion of coke in the catalyst regenerator is separately removed from the regenerator. This flue gas, which may be treated to remove particulates and carbon monoxide from it, is normally passed into the atmosphere. Concern with control of pollutants in flue gas has resulted in a search for improved methods for controlling such pollutants, particularly sulfur oxides and carbon monoxide.
The amount of conversion obtained in an FCC cracking, operation is the volume percent of fresh hydrocarbon feed changed to gasoline and lighter products during the conversion step. The end boiling point of gasoline for the purpose of determining conversion is conventionally defined as 430.degree. . Conversion is often used as a measure of the severity of a commercial FCC operation. At a given set of operating conditions, a more active catalyst gives a greater conversion than does a less active catalyst. The ability to provide higher conversion in a given FCC unit is desirable in that it allows the FCC unit to be operated in a more flexible manner. Feed throughput in the unit can be increased, or alternatively a higher degree of conversion can be maintained with a constant feed throughput rate.
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur, usually termed "feed sulfur". It has been found that about 2-10% or more of the feed sulfur in a hydrocarbon feedstream processed in an FCC system is invariably transferred from the feed to the catalyst particles as a part of the coke formed on the catalyst particles during cracking. The sulfur deposited on the catalyst, herein termed "coke sulfur", is eventually cycled from the conversion zone along with the coked catalyst into the catalyst regenerator. Thus, about 2-10% or more of the sulfur in the hydrocarbon feed is continuously passed from the cracking zone into the catalyst regeneration zone in the coked catalyst. In an FCC catalyst regenerator, sulfur contained in the coke is burned along with the coke carbon, forming gaseous sulfur dioxide and sulfur trioxide, which are conventionally removed from the regenerator in the flue gas.
Most of the feed sulfur does not become coke sulfur in the cracking reactor. Instead, it is converted either to normally gaseous sulfur compounds such as hydrogen sulfide and carbon oxysulfide, or to normally liquid organic sulfur compounds. These sulfur compounds are carried along with the vapor products recovered from the cracking reactor. About 90% or more of the feed sulfur is thus continuously removed from the cracking reactor in the stream of processed, cracked hydrocarbons, with about 40-60% of this sulfur being in the form of hydrogen sulfide. Provisions are conventionally made to recover hydrogen sulfide from the effluent from the cracking reactor. Typically, a very-low-molecular-weight off-gas vapor stream is separated from the C.sub.3+ liquid hydrocarbons in a gas recovery unit, and the off-gas is treated, as by scrubbing it with an amine solution, to remove the hydrogen sulfide, Removal of sulfur compounds such as hydrogen sulfide from the fluid effluent from an FCC cracking reactor is relatively simple and inexpensive compared to removal of sulfur oxides from an FCC regenerator flue gas by conventional methods. Moreover, if all the sulfur which must be recovered from an FCC operation could be recovered in a single recovery operation performed on the reactor off-gas, the use of two separate sulfur recovery operations in an FCC unit could be obviated.
It has been suggested to diminish the amount of sulfur oxides in FCC regenerator flue gas by desulfurizing a hydrocarbon feed in a separate desulfurization unit prior to cracking or to desulfurize the regenerator flue gas itself, by a conventional flue gas desulfurization procedure, after removal from the FCC regenerator. Clearly, both of the foregoing alternatives require elaborate, extraneous processing operations and entail large capital and utilities expenses.
If sulfur normally removed from the FCC unit in the regenerator flue gas as sulfur oxides is instead removed from the cracking reactor as hydrogen sulfide along with the processed cracked hydrocarbons, the sulfur thus shifted to the reactor effluent is then simply a small addition to the large amount of hydrogen sulfide and organic sulfur already invariably present in the reactor effluent. The small added expense, if any, of removing even as much as 5-15% more hydrogen sulfide from an FCC reactor off-gas by available means is substantially less than the expense of separate feed desulfurization or flue gas desulfurization to reduce the level of sulfur oxides in the regenerator flue gas. Hydrogen sulfide recovery systems used in present commercial FCC units normally have the capacity to remove additional hydrogen sulfide from the reactor off-gas. Present off-gas hydrogen sulfide recovery facilities could normally handle any additional hydrogen sulfide which would be added to the off-gas if the sulfur normally in the regenerator flue gas were substantially all converted to hydrogen sulfide in the FCC reactor off-gas. It is accordingly desirable to direct substantially all feed sulfur into the fluid cracked products removal pathway from the cracking reactor and reduce the amount of sulfur oxides in the regenerator flue gas.
It has been suggested, e.g., in U.S. Pat. No. 3,699,037, to reduce the amount of sulfur oxides in FCC regenerator flue gas by adding particles of Group IIA metal oxides and/or carbonates, such as dolomite, MgO or CaCO.sub.3, to the circulating catalyst in an FCC unit. The Group IIA metals react with sulfur oxides in the flue gas to form solid sulfur-containing compounds. The Group IIA metal oxides lack physical strength, and regardless of the size of the particles introduced, they are rapidly reduced to fines by attrition and rapidly pass out of the FCC unit with the catalyst fines. Thus, addition of dolomite and the like Group IIA materials must be continuous, and large amounts of material must be employed, in order to reduce the level of flue gas sulfur oxides for any significant period of time.
It has also been suggested, e.g., in U.S. Pat. No. 3,835,031, to reduce the amount of sulfur oxides in an FCC regenerator flue gas by impregnating a Group IIA metal oxide onto a conventional silica-alumina cracking catalyst. The attrition problem encountered when using unsupported Group IIA metals is thereby reduced. However, it has been found that Group IIA metal oxides, such as magnesia, when used as a component of cracking catalysts, have an undesirable effect on the activity and selectivity of the cracking catalysts. The addition of a Group IIA metal to a cracking catalyst results in two particularly noticeable adverse consequences relative to the results obtained in cracking without the presence of the Group IIA metals: (1) the yield of the liquid hydrocarbon fraction is substantially reduced, typically by greater than 1 volume percent of the feed volume; and (2) the octane rating of the gasoline or naphtha fraction (75.degree.-430.degree. F boiling range) is substantially reduced. Both of the above-noted adverse consequences are seriously detrimental to the economic viability of an FCC cracking operation and even complete removal of sulfur oxides from regenerator flue gas would not compensate for the losses in yield and octane which result from adding Group IIA metals to an FCC catalyst.
Alumina has been a component of many FCC and other cracking catalysts, but primarily in intimate chemical combination with silica. Alumina itself has low acidity and is generally considered to be undersirable for use as a cracking catalyst. The art has taught that alumina is not selective, i.e., the cracked hydrocarbon products recovered from an FCC or other cracking unit using an alumina catalyst would not be desired valuable products, but would include, for example, relatively large amounts of C.sub.2 and lighter hydrocarbon gases.
The conventional type of FCC catalyst regeneration currently used in most systems is an incomplete combustion mode. In such systems, referred to herein as "standard regeneration" systems, a substantial amount of coke carbon is left on regenerated catalyst passed from the FCC regeneration zone to the cracking zone. Typically, regenerated catalyst contains a substantial amount of coke carbon, i.e., more than 0.2 weight percent carbon, usually about 0.25 to 0.45 weight percent carbon. The flue gas removed from an FCC regenerator operating in a standard regeneration mode is characterized by a relatively high carbon monoxide/carbon dioxide concentration ratio. The atmosphere in much or all of the regeneration zone is, over-all, a reducing atmosphere because of the presence of substantial amounts of unburned coke carbon and carbon monoxide.
In general, reducing the level of carbon on regenerated catalyst below about 0.2 weight percent has been difficult. Until recently, there has been little incentive to attempt to remove substantially all coke carbon from the catalyst, since even a fairly high carbon content has had little adverse effect on the activity and selectivity of amorphous silica-alumina catalysts. Most of the FCC cracking catalysts now used, however, contain zeolites, or molecular sieves. Zeolite-containing catalysts have usually been found to have relatively higher activity and selectivity when their coke carbon content after regeneration is relatively low. An incentive has thus arisen for attempting to reduce the coke content of regenerated FCC catalyst to a very low level, e.g., below 0.2 weight percent.
Several methods have been suggested for burning substantially all carbon monoxide to carbon dioxide during regeneration, to avoid air pollution, recover heat and prevent afterburning. Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regeneration have been: (1) increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either (2) increasing the average operating temperature in the regenerator or (3) including various carbon monoxide oxidation promoters in the cracking catalyst to promote carbon monoxide burning. Various solutions have also been suggested for the problem of afterburning of carbon monoxide, such as addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat of combustion of carbon monoxide.
When using carbon monoxide combustion promoters heretofore commercially available, such as Group VIII noble metals in FCC catalysts, to provide complete carbon monoxide combustion, it has been found quite difficult to maintain the amount of coke on regenerated catalyst at an acceptably low level. Promoter materials when added to the catalyst have been found to provide adequate Co combustion in many cases, but have not been well accepted commercially because of the resulting high level of coke on regenerated catalyst, which has lowered conversion.
Complete combustion systems using an unusually high temperature in the catalyst regenerator to accomplish complete carbon monoxide combustion are also not altogether satisfactory. Some of the heat generated by carbon monoxide combustion is lost in the flue gas, because CO combustion takes place essentially in a dilute catalyst phase in an after-burning mode, and high temperatures can permanently adversely affect the activity and selectively of the FCC catalyst.
Several types of addition of Group VIII noble metals and other carbon monoxide combustion promoters to FCC systems have been suggested in the art. In U.S. Pat. No. 2,647,860 it is proposed to add 0.1-1 weight percent chromic oxide to an FCC catalyst to promote combustion of carbon monoxide to carbon dioxide and to prevent afterburning. U.S. Pat. No. 3,364,136 proposes to employ particles containing a small pore (3-5 A.) molecular sieve with which is associated a transition metal from Groups IB, IIB, VIB, VIIB and VIII of the Periodic Table, or compounds thereof, such as a sulfide or oxide. Representative metals disclosed include chrominum, nickel, iron, molybdenum, cobalt, platinum, palladium, copper and zinc. The metal-loaded, small-pore zeolite may be used in an FCC system in physical mixture with cracking catalysts containing larger-pore zeolites active for cracking, or the small-pore zeolite may be included in the same matrix with zeolites active for cracking. The small-pore, metal-loaded zeolites are supplied for the purpose of increasing the CO.sub.2 /CO ratio in the flue gas produced in the FCC regenerator. In U.S. Pat. No. 3,788,977, it is proposed to add uranium or platinum impregnated on an inorganic oxide such as alumina to an FCC system, either in physical mixture with FCC catalyst or incorporated into the same amorphous matrix as a zeolite used for cracking. Uranium or platinum is added for the purpose of producing gasoline fractions having high aromatics contents, and no effect on carbon monoxide combustion when using the additive is discussed in the patent. In U.S. Pat. No. 3,808,121 it is proposed to supply large-size particles of a carbon monoxide combustion promoter in an FCC regenerator. The smaller-size catalyst particles are cycled between the FCC cracking reactor and the catalyst regenerator, while, because of their size, the larger promoter particles remain in the regenerator. Carbon monoxide oxidation promoters such as cobalt, copper, nickel, manganese, copper chromite, etc., impregnated on an inorganic oxide such as alumina are disclosed for use. Belgian patent publication No. 820,181 suggests using catalyst particles containing platinum, palladium, iridium, rhodium, osmium, ruthenium or rhenium or mixtures or compounds thereof to promote carbon monoxide oxidation in an FCC catalyst regenerator. An amount between a trace and 100 ppm of the active metal is added to FCC catalyst particles by incorporation during catalyst manufacture or by addition of a compound of the metal to the hydrocarbon feed to an FCC unit using the catalyst. The publication notes that addition of the promoter metal increases coke and hydrogen formation during cracking. The catalyst containing the promoter metal can be used as such or can be added in physical mixture with unpromoted FCC cracking catalyst.
Applicant's employer and/or affiliates thereof purchased quantities of particulate additives from catalyst manufacturers. The additives were sold by the manufacturers for the purpose of introducing the additives into circulation in admixture with FCC catalyst in FCC units to promote combustion of carbon monoxide during catalyst regeneration in the units. Applicant's employer and/or affiliates thereof used the additives in their commercial FCC operations. One such additive was understood to contain a mixture of platinum-alumina particles and silica-alumina particles.